Almost all of our knowledge of drug receptors comes from in vitro experiments. However, it is important to study receptors in vivo for many reasons. Accordingly, one of our goals is to develop ligands and approaches for studying drug receptor sites in human populations by imaging techniques. Prior to imaging in humans, ligands need to be developed and tested in vitro and in vivo in animals. In our structure-activity studies of the cocaine receptor, RTI-55 was identified as a very potent compound with a high affinity for the dopamine transporter. It also has some affinity for the serotonin transporter as well. Accordingly, our previous publications show that RTI-55 is an excellent PET and SPECT ligand for studying dopamine transporters in vivo. It has been patented, licensed and is currently being used in several centers as an imaging reagent for diagnosing Parkinson's disease. Because of its significant affinity for serotonin transporters, it is possible to use RTI-55 as an in vivo binding ligand to study the occupancy of serotonin transporters by relevant drugs. We have examined the important newer antidepressant drugs which have a selective affinity for the serotonin transporter. Radiolabeled RTI-55 was administered and in vivo competition was carried out with fluoxetine, paroxetine and sertraline. At behaviorally effective doses, it was clear that these drugs occupied the serotonin transporter. Also, fluoxetine which is known to have a longer half-life in human subjects, had a much longer half-life in these animal studies than the other compounds. These results indicate that RTI-55 can be used to study the serotonin transporter, in particular to identify drugs that bind to that site, to determine the relative rate of occupancy, and also determine the duration of action of compounds at this site. We have previously utilized radiolabeled WIN-35,428 as a PET ligand to bind to dopamine transporters in vivo. In a recent study, we examined the kinetics and pharmacology of this binding in animals and humans. We clearly show that this compound is a promising radioligand for future studies of neuropsychiatric disorders that involve the dopamine transporter site.